1. Field of the Invention
The present invention relates to transducers for sensing the instantaneous pressure in an engine combustion chamber and, more specifically, the present invention relates to transducers for sensing the instantaneous pressure in an engine combustion chamber where an aluminum coupling disc is disposed between a diaphragm and a sensing element to improve performance. In an alternate embodiment, an aluminum expansion ring is adapted to support the sensor element within the transducer body.
2. Description of the Prior Art
Combustion chamber pressure transducers have found particular application in reciprocating internal combustion engines for detecting the pressure history occurring within the combustion chamber especially for detection of combustion anomalies such as detonation or knock, misfire or pre-ignition. Pressure transducers of this type are typically mounted through an aperture in the combustion chamber such as a threaded hole in the engine cylinder head and are employed to produce an electrical signal indicative of real time combustion pressures which is provided to an engine control comparator for use in controlling various engine parameters such as ignition timing, especially for controlling retarding of the ignition spark timing to prevent knocking. Various types of knock sensors which rely on sensing the high frequency vibration of the engine cylinder block have become a critical element in the management of spark ignition timing in internal combustion engines for optimizing efficiency, power and emission.
Heretofore, laboratory knock sensors or transducers for internal combustion engine usage have employed a metal diaphragm exposed to the combustion chamber gasses which moved an intermediate member to provide a force against a sensing element to generate an electrical signal indicative of the combustion pressure on the pressure responsive diaphragm. The prior art required a comparatively long intermediate member to prevent the extreme thermal environment from damaging the sensing element. This resulted in a transducer which did not have the necessary rapid response time to properly track the combustion event due to detonation or knocking and, thus, the transducer was not suitable for providing an electrical signal to the engine control computer for control of ignition timing. The vibration sensing technology used in prior art systems cannot accurately measure the combustion pressure events that occur in an engine combustion chamber and specifically are subject to drift due to thermal transients.
U.S. Pat. No. 5,559,280 the disclosure of which is hereby incorporated by reference, discloses a transducer housing for mounting in a port in a combustion chamber having a metal diaphragm exposed to the combustion gasses. The pressure of the combustion gasses deflects the diaphragm which locally deflects a simply supported beam member. The beam member is formed of refractory material having epitaxially grown crystalline piezoresistors formed on one surface of the beam and connected in a bridge circuit. Noble metal foil leads connect to the piezoresistors to form the bridge and provide attachment pads. The pads are noble metal welded to other noble metal foil pads provided on a ceramic substrate to form a sensor subassembly for mounting adjacent the diaphragm to form a transducer.
It would be desirable to provide a cylinder combustion pressure transducer that exhibits an increased thermal stability for improved sensing accuracy over a wide range of engine operating conditions. The extremely high temperatures of the combustion event introduce extreme temperature gradients and high heat loads to the face and internal parts of the transducer. The high temperature adversely affects the preload applied on the diaphragm and other internal parts on the sensing element resulting in unreliable performance especially during thermal transients.
The present invention provides a means to improve the thermal stability of a combustion pressure transducer by utilizing a coupling disc having a relatively high rate of thermal expansion relative to the other components in the transducer which is disposed between the sensor diaphragm and the sensing element. The sensing element is mounted on a mounting disc which is supported in a transducer body using a support ring. Thus, during thermal transients especially with increasing temperature, the coupling disc expands faster than the other components to maintain the preload on the sensing element. In an alternate embodiment, an aluminum expansion ring replaces the steel support ring and a prior art cup diaphragm is used to load the sensing element. The expansion ring functions to maintain the preload between the cup diaphragm and the sensing element by expanding and forcing the sensing element upward toward the cup diaphragm under increasing thermal loads.
One provision of the present invention is to provide a combustion pressure transducer having improved thermal operating characteristics.
Another provision of the present invention is to provide a transducer having a diaphragm acting upon,a coupling element having a high coefficient of thermal expansion which in turn acts upon a force sensing element to provide an accurate electrical signal responsive to combustion chamber pressures.
Another provision of the present invention is to provide a simply supported beam having piezoresistors formed thereon for localized deflection by a diaphragm exposed to combustion chamber pressures operating through a coupling element having a high coefficient of thermal expansion to rapidly generate an electrical signal indicative of changes in combustion chamber pressure.
Another provision of the present invention is to provide a combustion pressure transducer having improved thermal characteristics where an aluminum coupling element is used to transfer the deflection of a diaphragm which is exposed to combustion pressures to a piezoresistive sensing element.
Still another provision of the present invention is to provide an expansion ring having a relatively high rate of thermal expansion secured in the body of a combustion pressure transducer to provide support to a sensing element which contacts a diaphragm which is exposed to combustion pressures.